1. Field of the Invention.
The invention relates to optical memory systems and more particularly to methods of 2D spatial encoding of information for high-density writing in one- or multilayer optical, especially fluorescent data storage systems.
2. Description of the Prior Art.
The available optical memory systems utilize 2D carriers, generally with one or two information layers. Most of the prior art in the field of optical information recording are based on generating changes in the intensity of reflected laser radiation in local microregions (pits) of the information layer. These changes contain stored information and can result from interference effects on a microrelief surface of optical discs of a CD- Read-Only-Memory (ROM)-type, burning of holes in the metal film, dye bleaching, local melting of dye-containing polymers in widely used CD-Write-Once-Read-Many (WORM) systems, change in reflection coefficient in phase-change CD-Rewritable (RW) systems, etc.
FIG. 1 schematically presents geometry of two-dimensional spatial distribution of information pits on the surface of CD and DVD optical information carriers. Their spatial distribution in CD- and DVD-ROM can be characterized by such parameters as typical pit sizes (shortest pit length l, width w, depth d, and track pitch p) and channel bit length.
Numerical values of these and other CD- and DVD-ROM parameters are given in Table 1. [xe2x80x9cInformation Storage Materials xe2x80x9d, pp. 36, 42].
As can be seen from Table 1, transfer to the DVD-format considerably increases the density and consequently the volume of stored information. However, as can be seen from FIG. 1 and Table 1, information pits occupy yet only part of the information layer area which significantly reduces the values of density and volume of stored information as compared to maximum permissible magnitudes.
To increase the writing density such methods are used as transfer to shorter-wavelength radiation sources in combination with high-NA objective lens (see Table 1 and as an example [I. Ichimura et al, SPIE, 3864, 228]), a reduced track pitch and increasing the groove depth of the land groove recording type optical disk [S. Morita et al, SPIE, 3109, 167]. For high-density data storage, new media and techniques for data reading therefrom [T. Vo-Diny et al, SPIE, 3401, 284], pit-depth modulation [S. Spielman et al, SPIE, 3109, 98], and optical discs having square information pits arranged in symmetrical patterns [Satoh et al, U.S. Pat. No 5,572,508].
Data writing density as high as more than several terabits per cubic centimeter can be ensured by three-dimensional (monolithic) photosensitive media exhibiting various photophysical or photochemical non-linear effects at two-photon absorption. The best reading/writing mode for such 3D WORM or WER information carriers is cooperative two-photon absorption by photosensitive components and photoreaction products themselves via an intermediate virtual level as in the case of photochromic [D. Parthenopoulos et al, Science, 245, (1989), 843] or photobleacing [P. Cheng et al, Scanning, 18, (1996), 129] materials or registration of a change in refractive index as in the case of photorefractive crystals [Y. Kawata et al, Opt. Lett., 23, (1998), 756] or polymers [D. Day et al, SPIE, 3864, 103] and photopolymers [R. Borisov et al, Appl. Phys., B 67, (1998), 1].
Generally, such reading/writing mode allows local recording of data as marks (pits) analogous to information pits in conventional CD- or DVD-ROM, with varied optical properties in the volume of information medium.
Practical realization of this principle however is impeded by the large overall size needed for such writing of femtosecond laser radiation sources and the extremely low photosensitivity of the media themselves. The latter is dictated predominantly by the low magnitudes of cross sections of two-photon absorption currently known.
That same reason rules out the application of 1-10-mW small-size semiconductor lasers for two-photon data writing. Besides, the design of the 3D system based on this principle is rather complicated.
To increase the volume of information stored the application of multilayer bilateral information carriers would be technically more justifiable. However, they impose certain restrictions on the design and properties of the recording medium, data reading and writing procedures, especially in the depth, thereby creating additional difficulties.
In the reflection mode operation, each information layer of the multilayer optical data carrier must have a partially reflecting coat. This attenuates the intensity of both reading and reflecting, information-carrying, beams as a result of direct and reverse motion through the carrier up to a specified information carrying layer and back to the photoreceiver. In addition, as both beams are coherent, they are prone to difficult-to-read diffraction and interference distortions on pits and grooves of the information carrying layers occurring on their way.
In this case, preference should be given to multilayer optical information carriers with fluorescent reading that need no partially reflecting coats. Said information carriers ensure considerably reduced diffraction and interference distortions due to the noncoherence of fluorescent radiation, longer wavelength thereof in contrast to reading laser radiation as well as transparence and homogeneity (identity of refraction indices for some layers) of the optical medium with respect to the incident laser and fluorescent radiations. Consequently, multilayer fluorescent optical information carriers have advantages over reflecting ones. In addition, fluorescent reading enables a higher signal-to-noise ratio as compared to the absorption method.
Currently the general demand to all types, in particular fluorescent, of multilayer information carriers as optical discs and cards, tapes or cylinders is that they must ensure maximum possible volume and density of recorded information and maximum possible data reading rate. These requirements are met by minimizing the size of information pits and increasing the recording density thereof in each separate information layer while increasing the total number of layers, as well as by switching over to shorter-wavelength optical radiators as the information density storable in N-dimensional memory systems (N=1, 2, 3) is inversely proportional to the wavelength to the power N. However reducing the size of information pits and accordingly increasing the writing density thereof may lead to a lower intensity of the reading information signal and higher crosstalks due to the xe2x80x9cexcitationxe2x80x9d of the adjacent information layers the reading radiation is passing through. As a result, the reading signal-to-noise ratio goes down.
The purpose of the present invention is to eliminate the above drawbacks through application of a new method of spatial encoding in information layers and parallel data reading therefrom.
The subject of the present invention is a new ETT (xe2x80x9ceight-to-tenxe2x80x9d) method of two-dimensional spatial encoding of information stored in two- or three-dimensional, in particular fluorescent optical carriers. The method specifically ensures the same writing density as DVD carriers with EFM (xe2x80x9ceight-fourteen modulationxe2x80x9d) modulation code (see Table 1) but for a 0.8xc3x970.4 xcexcm information pit (fluorescent mark), i.e. as in CD data carriers. The largerxe2x80x94as compared to the DVD formatxe2x80x94pit size enables a simpler technology for manufacturing fluorescent multilayer carriers, for instance of ROM type, and a stronger fluorescent signal in reading. The high writing density is ensured through virtually 100% filling of the information layer area with fluorescent marks in a gap-free manner. The proposed method allows application of the parallel data reading procedure and a ten-fold higher reading speed than in DVD systems. Increasing the size of the channel bit to 0.4 xcexcmxe2x80x94which is 1.5 and 3 times higher than for CD and DVD formats, respectivelyxe2x80x94allows a significant reduction in the frequency band and hence in photoreceiver noises.
For equal values of the reading radiation wavelength and numerical aperture of the objective lens used, the proposed ETT method of two-dimensional encoding in fluorescent carriers enables a significantly lower magnitude of reading error probability in contrast to existing optical information carriers of DVD-type.
The proposed method is also applicable to other one- and multilayer optical data carriers based on various physical and chemical principles of forming information pits, such as photorefractive crystals and polymers, photopolymers, magnetooptical, phase-change, persistent spectral-hole-burning recording systems, as well as to other one- and multilayer optical data carriers of ROM-, WORM- and WER- type.
Said method is applicable to various forms of optical memory, for example, as an optical disc, optical memory card, optical memory tape or drum (cylinder), etc.
The subject of the invention is writing of one information byte in a field (microregion) consisting of ten (2xc3x975) square elements (xe2x80x9c(2xc3x975)-fieldxe2x80x9d) of specific size in which each said square element contains or does not contain any changes (different from those layer regions that do not carry information) in optical properties (absorption and reflection factors, refraction index, birefringence factor, etc.) testifying either presence or absence of an information pit therein. One information byte has an area of 10S, where S=axc3x97a is the area of one square element and a is the square side. Adjacent bytes are positioned on the plane close to each other without gaps.
Another subject of the invention is a representation of all 256 combinations composing an information byte on the information layer plane by xe2x80x9c(2xc3x975)-fieldsxe2x80x9d of two types wherein the first 222 combinations are represented by fields in which each information-carrying square element (pit) has inside a xe2x80x9c(2xc3x975)-fieldxe2x80x9d at least one identical adjacent element positioned transversely or lengthwise while each square element comprising no information pit has inside the field identical adjacent element (parity condition).
With appropriate modification, the method can be also utilized for three-dimensional volumetric data encoding as volumetric bytes recorded within a specified microvolume using the two-photon procedure, each said byte consisting of N number of cubic elements of certain size. Data can be read by either one- or two-photon procedure.
A further subject of the invention is the use for writing CandD information of the remaining 52 combinations each represented by one of two complementary xe2x80x9c(2xc3x975)-fieldsxe2x80x9d, wherein the parity condition can be violated only for the top left or bottom left square element of the field consisting of (2xc3x975) square elements.
The subject of this invention is the selection while writing from the pair of xe2x80x9c(2xc3x975)-fieldsxe2x80x99 of such a xe2x80x9c(2xc3x975)-fieldxe2x80x9d that when joined to the left field thereof enables meeting the parity condition inside each lengthwise strip consisting of joined to one another fields (bytes). In this case, the minimal regions of the information layer containing any changes (distinct from the layer regions containing no information) in optical (for example, fluorescent) properties, are composed of two adjacent square elements with varied optical (for example, fluorescent) properties (i.e. information pits (or fluorescent marks)) and consequently measure axc3x972a. The layer""s minimal regions free of information pits have the same size.
One more subject of the present invention is a possibility of gap-free filling of virtually entire area of the data-carrying layer with the proposed ETT-coded information pits.
Further, the invention concerns parallel data reading from an optical carrier with the ETT code by means of one- or two-dimensional photodetector array, for instance CCD cameras, enabling mutual longitudinal (and transverse, if necessary) motion relative to each other at a rate timed with both the size of the channel bit and the frame operating rate of the photodetector (CCD camera) array. In so doing, the adjacent element pairs in each transverse column belonging to different xe2x80x9c(2xc3x975)-fieldsxe2x80x9d are identified simultaneously. Both elements are considered information-carrying pits when the signals arrived from individual photodetectors xe2x80x9ccoveringxe2x80x9d respective square elements of the information layer exceed some level L1 and are not considered such when both signals do not exceed some level L2 less than L1. When the above-mentioned terms are not met, the square element with a higher signal level is identified as an information pit, while the other element, accordingly, is not considered an information pit. The L1 and L2 magnitudes are preset by the technical parameters of both the information-carrying medium and the optical reading device.
Another subject of the invention is an optical memory system of ROM type based on a multilayer fluorescent optical card and parallel data reader switching on reading radiation with a wavelength such that it excites fluorescence of the information pits, a dichroic mirror transmitting reading and reflecting fluorescent (information-carrying) radiation, an optical system shaping requisite spatial configuration of the reading beam in the location of a given information layer and fluorescent image thereof in the plane of the linear photodetector array (the linear array of 10 CCD cameras).
The optical card has 10 information layers of size 10 cmxc3x9710 cm, each of approximately 6 GB in capacity (or 60 GB in a card). Each layer consists of 250 information strips 400 xcexcm wide and is provided with CandD ( less than  less than control and display greater than  greater than ) information tracks allowing timing and autotracking of the linear CCD array as it moves across and along the optical card. Each of the CCD cameras has 1000xc3x971000 pixels of size 8 xcexcm and is capable of comparing a signal from each pixel with two levels, and signals from adjacent pixels between themselves. When the linear CCD array operates in the data pit-to-CCD pixel mode (at 20-fold magnification of the camera) at the rate of 30 frames per second, the whole linear array can read information at 3xc2x7108 pit/s, which in compliance with the ETT-code makes 30xc2x7106 B/s or approximately 220 Mb/s. In this case the rate of the linear CCD array is about 12 mm/s, which is almost 100 times slower than one-time data reading from a CD ROM optical disc.
The attached figures and examples illustrating the proposed invention will make its specific features and advantages more demonstrative.